Polyether polyols modified by polymers or copolymers of olefinically unsaturated monomers, so-called polymer polyols, and their use for the production of polyurethane plastics, particularly foams, are known. They are produced by the polymerization in situ of one or more vinyl monomers in standard polyether polyols. The use of acrylonitrile and mixtures thereof with styrene has acquired the greatest commercial significance, in the presence of a radical-forming polymerization initiator. The production and use of products such as these are described, for example, in U.S. Pat. Nos. 3,304,273; 3,383,351; Re-28715; Re-29118; 3,523,093; 4,104,236; 4,111,865; 4,119,586; 4,125,505; 4,148,840 and 4,172,825; and in German Pat. Nos. 1,222,669; 1,152,536 and 1,152,537.
Polyurethanes produced with polymer polyols of this type are distinguished by an improvement of their properties. In particular, the hardness and load bearing strength of flexible polyurethane foams are favorably influenced. Therefore it is possible to obtain relatively low densities and, thus, to save on starting material for the same level of hardness and tearing strength of previous polyurethanes.
In addition, the polymer polyols provide flexible foams with a greater open-cell character and, in doing so, counteract shrinkage of the fresh foams during storage. Finally, it is possible by means of the polymer polyols, provided that the starting polyether is suitably selected, to produce so-called highly elastic, cold-hardening foams. conventional processes for producing foams such as these, there is no need to use special polyisocyanates with adapted reactivity; instead it is possible to use standard commercial products, particularly the tolylene diisocyanate predominantly used in the production of flexible foams.
Ideally, the polymer polyols are relatively low-viscosity, finely divided, non-sedimenting dispersions of the polymer, preferably an acrylonitrile or acrylonitrile/styrene graft (co) polymer, in the substantially unchanged polyether polyol. Characteristic features of the quality and processibility of the polymer polyols are low viscosity, stability in storage (resistance to sedimentation) and particle size. These properties are influenced primarily by the type of starting materials used and by the quantitative ratios between them. For monomer mixtures of acrylonitrile and styrene, optionally together with small quantities of other comonomers, the optimum properties of the polymer polyol (as low a viscosity as possible; absence of sediment and agglomerate; small particle size), for a given molecular weight of the starting polyether, lies within a relatively narrow range of production parameters. The monomer content of the mixture and the monomer ratio both have a particularly marked influence upon the quality of the end product. Starting out from a polymerization mixture containing pure acrylonitrile, the viscosity, particle size and agglomerate content pass through a minimum with increasing styrene content of the mixture and rise sharply with increasing styrene content beyond this minimum. They also increase drastically with increasing monomer total, based on the starting polyether. However, the abovementioned values also increase with decreasing molecular weight of the starting polyether and also with a reduction in the polymerization temperature to below 100.degree. C.
The polymer polyol dispersions are stabilized against sedimentation by the incorporation of some of the molecules of the starting polyether into the polymer formed in situ. It may be assumed that the reaction conditions influence the grafting frequency so that it is only at the optimum of the parameter range that it is possible to obtain maximum grafting frequency which guarantees the stability in storage and the processibility of the product. If the limits of this range of parameters are exceeded, increased viscosity and coarsening of the particles in the polymer polyol to the point of agglomeration and sedimentation are the inevitable consequences. The use of polyethers having a short chain length, equivalent weight less than 1000, also leads to highly viscous, coarse suspensions.
There is no technical teaching in the existing literature to show how these limitations, to which the process for producing polymer polyols is subject, can be overcome and how the properties of the end product can be improved, even in the case of mixtures which are critical in regard to viscosity and particle size.
It would be desirable for example to obtain a higher solids content, irrespective of the molecular weight of the starting polyether, in order to increase further the property-improving effect of the polymer polyol and to make it possible for the processor to blend the product with other polyols adapting the requirements to the properties of his polyurethane foams. At the same time, however, the processibility of the product should not be adversely affected. In other words neither viscosity nor particle size should be increased too greatly.
It has already been proposed to use standard molecular weight regulators and telogens in the in situ polymerization reaction in order to reduce the viscosity of polymer polyols in critical mixtures. However, this procedure has not yet been successful because these substances, for example the mercaptans normally used for polymerization purposes, compete with the polyether polyol as transfer agents with a high transfer constant and in fact reduce the grafting yield.
Although the quality of the end product can be improved to a certain extent by increasing the concentration of initiator, there are limits to this process. Increased additions of peroxide involve the danger of an oxidative attack on the polyether. This promotes degradation and cross-linking reactions. At the same time, secondary products formed can give rise to core discoloration in the production of foams. A toxic secondary product is formed from azoisobutyronitrile (AIBN), which has been successfully used in practice, so that in this case, too, the concentration of initiator should be kept as low as possible.
New developments in the processing technology of flexible polyurethane foams, particularly in the upholstery and automobile fields, have created a demand for flame laminatability and high frequency ("HF") weldability of flexible polyether-based foams with other materials, particularly textiles. However, commercially available flexible polyether urethane foams cannot be subjected to high-frequency welding. There has been no shortage of attempts to make them suitable for HF-welding by the incorporation of suitable additives, primarily substances having a high dielectric constant.
In particular, it would seem to be desirable to provide the foam manufacturer with ready-formulated starting materials from which HF-weldable foams can be produced without any need for further additives.
Numerous free radical polymerizable, ethylenically unsaturated compounds are described in the existing patent literature as being suitable for the production of polymer polyols. However, apart from the polyether polyols modified by the polymerization of acrylonitrile or styrene/acrylonitrile mixtures, no other products have as yet acquired any commercial significance. This is in spite of the fact that small quantities of other monomers may be combined with styrene and acrylonitrile without the properties of the end product being significantly altered.
In the known Patents, for example German Pat. No. 1,222,669, reference is made to the use of .alpha.,.beta.-unsaturated monocarboxylic and polycarboxylic acids, although products on this basis distinguished by particular properties or by improved processibility have not yet been described. In particular, it is not apparent from the existing literature that the presence of .alpha.,.beta.-unsaturated carboxylic acids during the polymerization reaction would make it possible for the stability problems referred to above to be solved and the process limits to be extended.
The object of the present invention is to provide a process for the production of polymer polyols which may be applied more universally than those previously known. The process of the invention shows improvements in the size of the monomer content, the monomer ratio and the choice of the starting polyether and, in addition, gives products which may be converted into foams and show outstanding mechanical properties, and may also be HF-welded. The larger number of polyethers which may be converted into polymer polyols by the new process also makes it possible for new fields of application to be opened up for the class of products in question.